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Abstract Background Improved Samba Mahsuri (ISM) is an elite, high-yielding, bacterial blight resistant, fine-grained rice variety with low glycaemic index. It is highly sensitive to salt stress, particularly at seedling stage, which significantly reduces its yield potential in coastal areas. A salinity tolerant QTL, Saltol, associated with seedling stage tolerance was previously mapped on chromosome 1 (10.6–11.5 Mb) from the Indian landrace, Pokkali and is effective in different genetic backgrounds. The objective of this study was to enhance salinity tolerance of ISM by incorporating the Saltol QTL through marker-assisted backcross breeding using the breeding line, FL478 (Pokkali/IR29). Results Foreground selection was carried out at each generation using five Saltol-specific markers and three bacterial blight resistance genes, Xa21, xa13 and xa5. Background selection was conducted using 66 well distributed polymorphic SSR markers and at the BC3F2 generation, a single plant with maximum recurrent parent genome recovery (95.3%) was identified and advanced to the BC3F4 generation. Based on bacterial blight resistance, seedling stage salinity tolerance and resemblance to ISM, four advanced breeding lines were selected for testing in replicated experiments near Hyderabad, India. A promising near-isogenic line, DRR Dhan 58, was evaluated in multi-location trials-coastal salinity and it showed significant salinity tolerance, resistance to bacterial blight disease, high yield and excellent grain quality during the 2019 and 2020 trials. DRR Dhan 58 was 95.1% similar to ISM based on genotyping with the 90 K SNP chip. Whole genome resequencing analysis of Pokkali and FL478 which were salinity tolerant checks, ISM and DRR Dhan 58 showed a high degree of relatedness with respect to the candidate gene loci for Saltol and OsSKC1 (Shoot K + Concentration 1). Conclusion DRR Dhan 58, possessing Saltol and three bacterial blight resistance genes (Xa21, xa13 and xa5) in the genetic background of the Indian mega-variety of rice, Samba Mahsuri, was developed for potential cultivation in areas prone to seedling stage salinity, as well as areas with endemic bacterial blight disease. This entry had a 24% yield advantage over the recurrent parent ISM under coastal saline conditions in multi-location trials and was recently released for commercial cultivation in India.

Swarna-Sub1, a ruling submergence tolerant variety of rice, showed a significant decrease in the yield in major blast disease prone areas, which is caused by fungus Magnaporthe oryzae (Pyricularia oryzae). To overcome this problem, three major blast resistance genes viz., Pi1, Pi2, and Pi54 were pyramided through marker-assisted backcross breeding using donors, Swarna-LT (having Pi1 + Pi54) and Swarna-A51 (having Pi2). Foreground selection was carried out using molecular markers tightly linked to three blast resistance genes and also submergence tolerance for retaining the Sub1 gene in the recurrent parent. Based on disease resistance and agro-morphological performance of backcross-derived lines, we selected the best 25 plants of two- and three-gene pyramided BC3F2 homozygous lines with a maximum of ~ 93.5% recurrent parent genome and were further advanced to the BC3F5 generation. The stringent recurrent parent genome recovery analysis using SSR markers limited the linkage drag from a minimum of 0.2 Mb to a maximum of 2 Mb in all three-gene pyramided lines. The two and three blast resistance genes pyramided lines, i.e., SS30-24-82, SS30-24-73, and SS30-24-46, displayed a high level of blast resistance and submergence tolerance. The successful use of marker-assisted backcrossing strategy coupled with phenotypic selection helped in the development of Swarna-Sub1 lines having multiple blast resistance genes with superior agro-morphological and grain quality traits, which could serve as valuable genetic stocks in the rice breeding program. [ABSTRACT FROM AUTHOR]